Patient blood oxygen saturation parameter (SpO2) monitoring systems use light at different wavelengths transmitted through a blood-perfused portion of a patient e.g. a finger or ear lobe, to determine the oxygen saturation parameter of the patient. For example, light emitting diodes (LEDs), emitting light at the desired wavelengths, are arranged to be placed on one side of a patient finger and a photo-detector is arranged to be placed on the other side of the patient finger. The light from the LEDs is transmitted through the finger and detected by the photo-detector. The optical signals from the LEDs are modulated signals at a nominal predetermined carrier frequency. The electrical signal produced by the photo-detector, representing the received optical signal, is demodulated to produce signals representing estimates of the optical signals produced by the LEDs. These signals are processed in a known manner to calculate the oxygen saturation parameter for the patient.
There are several sources of noise interference which are introduced into the system. Electrical noise is introduced into the electrical portion of the SpO2 system by other electrical equipment being operated in the vicinity of the SpO2 system. Optical noise is introduced e.g. by the presence of ambient lighting in the patient room. It is to minimize the adverse affect of such noise that existing systems modulate the signal coupled to the LEDs at the carrier frequency. Subsequent processing involves subtracting adjacent samples, thus minimizing noise and reinforcing desired signals. Typically, however, the largest component of optical noise is introduced by ambient light detected by the photo-detector. Because the ambient light is generated by lights coupled to power mains, it has an AC component at the power mains frequency. This frequency is typically 50 or 60 Hz.
This AC noise component of the optical noise is demodulated. It is possible that harmonics of this interference signal are within the passband of the demodulated received signal and are eliminated or minimized by subtraction. Some existing systems determine a priori expected frequencies of such harmonics, and pre-select a modulation frequency which is expected to minimize the adverse affects of these harmonics. However, such a system having a pre-selected modulation frequency set for one power mains frequency (e.g. 50 Hz) is not be optimum if used with another power mains frequency (e.g. 60 Hz). Other existing systems attempt to detect the presence of such harmonic interference signals in the demodulated signal, and select one of a predetermined set of modulation frequencies which minimizes the adverse affects of the harmonics. This detection process is difficult and complicated, and it is possible that it does not always accurately identify a harmonic frequency. In either case, the frequency of the power mains, while generally at a nominally constant frequency, does vary. Such variation results in degrading the reduction of interference resulting from to the ambient noise.